1. Field
The present specification relates to a method for preparing slow release fertilizer with a coated surface. Furthermore, the present specification relates a formulation of the coating layer.
2. Technical Background
Lots of effort has been expended in preparing slow release or controlled release fertilizer. The focus areas are reducing initial dosage and keeping constant levels of fertilizer for extended durations. Mechanisms include controlled water solubility of the material (by semi-permeable coatings, occlusion, or by inherent water insolubility of polymers, natural nitrogenous organics, protein materials, or other chemical forms), by slow hydrolysis of water-soluble compounds, and diffusion of materials through porous structure or by biodegradability of carriers. Polymer coated fertilizer has been commercialized in many countries.
Lignin is the second most abundant natural polymer in the world, and is a waste product of pulping mills. Lignin does not dissolve in water but partially or totally dissolves in some solvents, such as ethanol, methanol, hexane and so on. Lignin dissolved in organic solvents exhibits good film-forming properties. Therefore, it can be easily coated on the surface of fertilizer particles.
In the Chinese patent publication CN1164531, lignin is dissolved in water by first adding 5-10% sodium dodecyl benzene sulfonate, and then the slow-release fertilizer is produced by mixing the lignin solution and fertilizer or spraying the lignin solution onto the fertilizer. The final drying method is hot air drying.
It is clear in claim 1 of the Chinese patent publication CN1164531 that the coating materials are powders of lignocellulosic biomass, such as wood sawdust, wood residues, branches, agricultural residue and walnut shells. In these kinds of powder, lignin is not the main element.
The Chinese patent publication CN1164531 uses other adhesives to adhere the powders to the surface of fertilizer.
In the two patent applications WO 11055174 and WO11005175, two methods related to lignin are disclosed. One is directly mixes lignin and urea. The other is directed to a preparation of lignin reacted with urea at high temperature in an aqueous system.